1. Field of the Invention
The present invention relates to a video image capturing and displaying method, and more particularly, to a video image capturing and displaying method used for a web camera.
2. Description of the Prior Art
Image information is an important form of media in modern society since we need to transmit images from one place to another, such as by downloading pictures or movies from the Internet, synchronous communication in net meetings, transmitting image data in a wireless manner, etc. Because image data typically has a large amount of bytes, transmission of image data is slow and inefficient. Therefore, video compression techniques are used before sending image data to improve the transmission efficiency.
Web cameras, a popular peripheral device for a computer, are used to record a series of dynamic images. The recorded video is compressed and passed to one computer by a transmission interface. Please refer to FIG. 1. FIG. 1 illustrates a well-known computer 10 equipped with a web camera. The computer 10 comprises a web camera 12, a USB transmission interface 14, and a terminator 16. The web camera 12 can take a plurality of pictures in a period, the pictures being transformed into video signals. The web camera 12 further comprises a compression circuit, which compresses the video signals. The video signals are sent to the terminator 16 by the USB transmission interface 14 that connects the web camera 12 and the terminator 16. The terminator 16, functioning as a well-known computer, communicates with the Internet.
A USB transmission interface has different transmission specifications with which to comply. Different specifications prescribe different transmission speeds. Here, we discuss the case of 1 M Bytes/sec transmission speed. When the web camera 12 captures dynamic pictures, the frame rate should be 30 frames per second so that people can see these discrete frames displayed in a period as a continuous video. Generally speaking, one frame taken by the web camera 12 has about 100 K pixels, wherein the horizontal resolution is 352 pixels and the vertical resolution is 288 pixels. Therefore, according to the frame rate of the web camera 12 and pixel numbers of each frame, the web camera 12 receives 30*100 K Bytes =3 M Bytes within one second. However, as the mentioned above, the USB transmission interface 14 merely provides transmission rate of 1 M Bytes/sec. So, the captured images should be compressed into one third or less so that they can be transmitted by the USB transmission interface 14.
Video compression is needed to make image transmission possible under limited speed of a transmission interface. There are two kinds of compression methods in the prior art: frame-based compression and block-based compression. Frame-based compression uses one frame as the basic image compression unit. In other words, all pixels in one frame are compressed under one compression mode in the frame-based compression. This method uses less hardware, is less complicated, and is more cost effective. However, each group of pixels in one frame has different complexity and if a group of pixels with high complexity is compressed with a low compression ratio, this still leads to too large data for transmission even after compression. In contrast, if the group of pixels belonging to a simple drawing is compressed with a high compression ratio, the image has serious distortion.
Block-based compression, taking one block of a frame as the basic image compression unit, compresses complicated image blocks with a higher compression ratio so that the volume of the compressed data is small enough, and compresses simple image blocks with a lower compression ratio so that the compressed data is more like the original image. Although block-based compression solves the problems of frame-based compression, it divides one frame into blocks and each block unit will be given one compression mode. Because the sensor input order is the line-based order, we need additional memory to store these blocks. Therefore, division of one frame increases system hardware and cost.